The present invention relates to a transmission device of a four-wheel drive vehicle. Particularly, the present invention relates to the transmission device, in which an engine driving force is transmitted from an input shaft to a hollow counter shaft while a rotational speed is changed. The engine driving force is further transmitted to an output shaft of either one of front and rear wheels inserted in a hollow portion of the counter shaft. At that time, the engine driving force is also transmitted to an output shaft of the other one of the front and rear wheels.
Conventionally, in the case of a manual transmission vehicle for space-saving, there is generally known a structure as follows. Namely, the engine driving force is transmitted from the input shaft to the hollow counter shaft while a rotational speed is changed, and the engine driving force is transmitted from the counter shaft to the output shaft of the front (rear) wheel and to the output shaft of the rear (front) wheel is disposed on the same rotation axis as the input shaft.
For example, Japanese Patent Unexamined Publication No. Hei. 7-167257 discloses a transmission device of a four-wheel drive vehicle that comprises an input shaft, a hollow counter shaft, a front wheel side output shaft, a drive gear and a driven gear. The input shaft receives the engine driving force. The hollow counter shaft is disposed in parallel to this input shaft to transmit the engine driving force for changing a rotational speed to a predetermined one. The output shaft of the front wheel is inserted in a hollow portion of the counter shaft to transmit the driving force to a final reduction gear of a front wheel. The drive gear is coaxially coupled to an end of the output shaft through a center differential device and a viscous coupling. The driven gear engages with the drive gear to change a rotation axis to the same rotation axis as the input shaft and to change the rotating shaft to the output shaft of the rear wheel
However, in the foregoing structure of the prior art, the center differential device and the viscous coupling are positioned on the same rotation axis as the counter shaft, that is, at a lower portion in a transmission case, and there is a problem that gears stir oil to produce useless resistance, and fuel efficiency is deteriorated.
Besides, in an automatic transmission vehicle, a main speed change is performed on the same shaft as the input shaft of the engine driving force. Such a transfer device of the four-wheel drive vehicle, as a center differential device or a viscous coupling is also disposed on the same shaft as the input shaft of the engine driving force. However, in the prior art, since the center differential and the viscous coupling are disposed on the same rotating axis as the counter shaft, it has been difficult to use the power distribution devices of the four-wheel drive vehicle for the automatic transmission vehicle and the respective mechanisms in common.
Further, when such an existing transfer mechanism as a hydraulic coupling for distributing the power between the front wheel and the rear wheel is adopted, an extensive change of layout is necessary for disposing at the same rotating axis as the counter shaft, and there is a problem that it requires a long development term.